During lactation in the rhesus monkey the ovaries are quiescent for about 5 months as indicated by low estradiol and progesterone measured every other day throughout this period. Menstruation is first observed at about 5 months following parturition. Resumption of ovarian cyclicity did not precede the first menstruation but began at a variable time thereafter. Unusual patterns of gonadotropin secretion were observed during lactation amenorrhea. LH levels were consistently higher than those observed in the non-ovulatory phases of the normal menstrual cycle. However, the levels were not as elevated as would be expected in ovariectomized animals. FSH levels, on the other hand, were either normal or slightly subnormal as compared to the menstrual cycle. It is concluded that suckling of the young inhibits gonadotropin secretion since gonadotro pin levels are lower than might be expected in a period of major gonadal hormone deficit. In addition, the ovaries appear to be partially refractory to the gonadotropin secreted since plasma levels of estrogen and progesterone remain low.